Technical Abstract:
Directed aerial displacement requires that an organism’s airspeed exceeds ambient wind speed. For biologically relevant altitudes, wind speed increases exponentially with increased height above the ground. Most insects thus disperse according to atmospheric conditions. However, those insects flying close to the Earth's surface are in the flight boundary layer where insect airspeeds are relatively high. Over the past 17 years, we have studied boundary-layer fliers by following individual insects as they migrate across the Caribbean Sea and the Panama Canal. Although most migrants evade drought or cold, nymphalid and pierid butterflies migrate across Panama near the onset of the wet season. Dragonflies of the genus Pantala migrate in October with frontal systems. Migrating the furthest and thereby being the most difficult to track, the diurnal moth Urania fulgens migrates from Central to South America. The butterflies and dragonflies are capable of directed movement towards a preferred compass direction in variable winds, whereas the moths drift with winds over water. The butterflies orient using both global and local cues. In accordance with optimal migration theory, butterflies and dragonflies adjust their flight speeds to maximize migratory distance per unit fuel, whereas moths do not. Moreover, only butterflies adjust their flight speed in accordance with endogenous fuel reserves. It is likely that the insects use optic flow to gauge their speed and drift, and thus must migrate where sufficient detail in the Earth’s surface is visible to them. The abilities of butterflies and dragonflies to adjust airspeed over water indicate their possession of sophisticated control and guidance systems pertaining to migration.